;inc：加 1
;dec：减 1
(define (+ a b)
    (if (= a 0)
        b
        (inc (+ (dec a) b))))

(define (+ a b)
    (if (= a 0)
        b
        (+ (dec a) (inc b))))
        
;第一个过程
(+ 4 5)
(inc (+ 3 5))
(inc (inc (+ 2 5)))
(inc (inc (inc (+ 1 5))))
(inc (inc (inc (inc (+ 0 5)))))
(inc (inc (inc (inc 5))))
(inc (inc (inc 6)))
(inc (inc 7))
(inc 8)
9
;所以这是递归的计算过程

;第二个过程
(+ 4 5)
(+ 3 6)
(+ 2 7)
(+ 1 8)
(+ 0 9)
9
;所以这是迭代的计算过程